1. Field of the Invention
The invention relates to processes for pelletizing ash resulting from the combustion of carbonaceous fuels such as coal and, more particularly, to preventing agglomeration and other material-handling flow problems that can occur during pellet curing.
2. Description of the Prior Art
The combustion of carbonaceous fuels for the production of electricity and/or process steam by the utility and industrial sectors is a major generator of ash. Apprehension about the pollution from the smoke stack industries and utilities has led to the implementation of flue gas cleaning technologies for sulfur dioxide (SO.sub.2) emissions, as well as particulate emissions. Technologies for the control of sulfur emissions for combustion systems have been developed and are being implemented. These technologies generically have been termed clean coal technologies ("CCT"). Examples of these CCT's are fluidized bed combustion ("FBC"), spray dryer flue gas desulfurization ("FGD"), limestone furnace injection ("LFI") and calcium-based sorbent duct injection ("CDI"). Each of these CCT's introduces lime (CaO) or limestone (CaCo.sub.3) to neutralize the SO.sub.2 emitted during combustion by reactions which form sulfate/sulfite salts.
Perhaps the most advanced of the CCT's is FBC. FBC involves the combustion of a carbonaceous fuel in a fluidized bed containing a sorbent, such as limestone, which is calcined to form lime. The calcined lime reacts with SO.sub.2 and O.sub.2 typically to form calcium sulfate. The control of sulfur emissions is accomplished within the combustion chamber, which eliminates the need for adding a wet flue gas desulfurization ("WFGD") process.
Dry flue gas desulfurization ("DFGD"), also known as the spray dryer process, also is possible for SO.sub.2 control. The spray dryer process is a flue gas cleaning process whereby a fine mist of lime or lime slurry contacts the flue gas in a reaction chamber, and produces a dry ash. Sulfur dioxide reacts with the mist and forms calcium sulfate/sulfite. Limestone/lime furnace injection, or LFI, involves the direct injection of pulverized calcium-based sorbent, such as hydrated lime or limestone, into a combustion zone above the burners. Once again, the SO.sub.2 in the flue gas reacts to form calcium sulfates/sulfites. Finally, the calcium duct injection process, or CDI, introduces powered sorbent lime or limestone into the flue gas ductwork downstream of the combustion zone, but upstream of a particulate collection system.
All of these coal-fired CCT's generate several high-volume residues. These include ash from the combustion of coal, as well as reaction products from sulfur oxide (SO).sub.2 control processes, including unreacted calcined sorbent. Ash is categorized as bottom ash, collected from the furnace, and fly ash, which consists of lighter, finer particles removed by mechanical collectors, electrostatic precipitators ("ESP"), and/or fabric filters downstream of the combustor. The neutralization of SO.sub.2 in the flue gas with calcium-based sorbent results in unreacted lime/limestone, thereby making CCT residue unique.
A particular problem arises with respect to FBC ashes. In part, this is because FBC ashes are distinctly different from ashes generated by other CCT technologies, as well as conventional pulverized fuel ash ("PFA") as produced by conventional pulverized coal combustion processes. FBC ashes are not glassy, like PFA ashes, due to a lower temperature of combustion (1500.degree. F., compared to 2500.degree.14 3000.degree. F.). Furthermore, FBC ashes appear to have a high amount of soluble components and generate high pH wastewaters, unlike those from pulverized fuel ashes. Another unique property of FBC ash is the exothermic character of the ash (the ability to generate heat upon the addition of water) and the swelling of the ashes upon contact with water. These unique properties of FBC ash require specialized processing and disposal techniques.
Processes typically used for the handling and disposal of PFA are inadequate for FBC ash. Typically, PFA is conditioned with water and is sluiced to a pond for disposal FBC ashes have been conditioned with water and have been trucked to a disposal site for compaction. Unfortunately, the addition of water to FBC ash generates considerable heat and steam, both of which adversely affect operations and compaction. In addition, the ashes cannot be transported great distances due to their tendency to harden in the trucks.
Alternatively, FBC ash could be handled in dry form. However, the dry-handling of ash requires the use of pneumatic transfer and pressurized transport equipment similar to that used in the cement industry. The dry-handling of ash is expensive, especially if it is desired to backhaul the ash to the coal mine which supplied the coal. Such a haul often is lengthy and is not compatible with the barge and rail service which supplied the coal, thereby limiting the economic advantages of backhaul.
Another process often employed for the handling of ash is pelletization. Pelletization is not new, and commercial processes exist for the pelletization of conventional PFA using cement or lime as a binder and as a chemical reactant. The gravel-like material produced by pelletization permits the use of conventional backhaul equipment, thereby enhancing the economics of ash disposal. In addition, the pelletized material is less prone to leaching of metals and other components of the ash due to the impermeable nature of the pellets. Another advantage of pelletization is that the pelletized form and the hardness of the pellets make the pellets amenable to use as an aggregate in various construction applications, such as road bases, concretes, and masonry units and shapes.
While the successful pelletization of FBC ashes is a desirable objective, acceptable commercial processes for the pelletization of FBC ashes have not been available. One approach that appears to show promise is disclosed in U.S. Pat. No. 4,344,796, issued Aug. 17, 1982 to L. John Minnick. In the '796 patent, residue from a fluidized bed combustor ("FBCR") is recovered from the combustion process and is mixed with water so as to convert all of the quicklime particles in the FBCR to calcium hydroxide. The '796 patent discloses that about 20-30% by weight of water is needed to hydrate substantially all of the quicklime particles. The water and FBCR are mixed for about 15 minutes so that hydration will be complete. Thereafter, the water-FBCR mixture is cooled to room temperature. The converted FBCR then is mixed with PFA or other pozzolanic material. The '796 patent discloses that the resultant mixture contains about 90-10% FBCR and 10-90% PFA. Additional water may be added to bring the moisture content of the mixture to within the range of about 9-20% by weight.
After the aforementioned mixture has been prepared, it is formed into desired shapes and then is cured over a period of about 7-28 days at a temperature of about 100.degree. F. The shaped and cured product is stated to be environmentally stable, that is, it can be stockpiled for long periods of time without excessive leaching or surface runoff. The shaped and cured product also can be used as a structural material (such as masonry blocks) or it may be crushed to form a high-strength aggregate that is useful in load-bearing applications.
Despite the promising approach taken by the '796 patent, certain concerns have not been addressed adequately. The most important concern relates to the type of fly ash that can be pelletized successfully. The '796 patent indicates that pelletizing is successful only with a mixture of FBCR, bituminous fly ash, and sodium silicate as a binder. Pellets made from FBCR and FBC fly ash, even with the addition of sodium silicate binder, could not be pelletized. The result of the failure to incorporate FBC fly ash into the pellets means that a separate source of pozzolanic material such as PFA is needed. Accordingly, the '796 patent does not disclose an integrated technique for disposing of all of the ash products (both FBCR and FBC fly ash) produced by a fluidized bed combustor. The inability to successfully pelletize and dispose of all of the ash products produced by a given fluidized bed combustor is a serious limitation on the technique disclosed by the '796 patent.
Another problem with the technique disclosed in the '796 patent is the tendency of the pellets to be sticky so as to agglomerate into a solid mass during curing. In addition, pelletization of FBCR creates large amounts of heat and steam, which, although good for the curing process, is a concern from a safety and process control perspective. It also has been found that the pellets containing unhydrated FBCR can become weak and lose strength during curing, resulting in the generation of large amounts of fines and dust. An important requirement of the pelletizing process is the intimate mixing of the ash and the water (typically by means of a high-intensity pin mixer). However, FBCR contains particles which are large and which can create excessive wear of the mixing equipment.
Another pelletizing technique is disclosed in U.S. Pat. No. 4,880,582, issued Nov. 14, 1989 to J. J. Spanjer et al. In the '582 patent, a mixture of fly ash, lime, water, and "other components" are provided. The "other components" are stated to be residues of combustion processes, such as bottom ash, ash from fluidized bed boilers, and other types of ash. The mixture is pelletized and is directed into a hardening reactor (curing silo) where the pellets are embedded in a fine-grained material having a water absorption of at least 8%. A preferred embedding material is fly ash produced by the combustion process. After the pellets have been placed in the curing silo with the embedding material, steam is added to maintain a temperature of between 85.degree. F. and 212.degree. F. Curing is carried out for about 16-18 hours. After curing, the pellets are screened. Excess embedding material is returned to the mixer for use as a starting material in the formation of additional pellets.
The '582 patent, like the '796 patent, fails to address certain concerns. By mixing the "other components" directly into the pelletizing mixture, it is believed that excessive heat and steam will be generated in the mixing equipment, and that the mixing equipment will be subjected to excessive wear. The process disclosed by the '582 patent also requires the use of a binder in the form of lime that is provided from a separate source. The process disclosed by the '582 patent additionally requires the use of steam in the curing silos to effect adequate curing. Furthermore, while the curing time is considerably less than that disclosed in the '796 patent, it still is longer than desired.
Desirably, a process would be available for the pelletizing of all of the FBCR and FBC fly ash produced by a given FBC without the need to (1) use a binder from an external source, (2) supply PFA or other pozzolanic material from a separate combustion source, or (3) supply steam or other curative agent. The processing technique preferably would avoid the agglomeration of the pellets during curing. Such a process preferably would minimize breakage of the pellets during curing and swelling of the pellets in the curing area. Also, such a process desirably would cure the pellets in a very short period of time. An additional feature desired of any pelletizing process would be the minimization of wear of the high-intensity mixing equipment necessary for intimate mixing of the ash with water for pelletization.